docs: document updateRtt side effect in ping method

[Copilot]

The public ping() method automatically updates the RTT repository on successful pings (added in commit 1dc4258), but this side effect was not documented in the API comments.

Changes

• Added documentation clarifying that ping() updates the RTT repository with measured round-trip time
• Updated .gitignore to exclude build artifacts (.venv, .vcpkg, .build)

/**
 * Performs a single ping on the given connection.
 * Opens a new stream for the ping.
 * On successful ping, automatically updates the RTT repository with the
 * measured round-trip time.
 * @param conn Connection to ping.
 * @param timeout Timeout for the ping operation.
 * @return RTT of the ping.
 */
CoroOutcome<std::chrono::microseconds> ping(
    std::shared_ptr<connection::CapableConnection> conn,
    std::chrono::milliseconds timeout);

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